E vents in th e action p o te n tial • Figure 7.5
Sodium (Na+) and
potassium (K+)
channels open and
close in sequence
during the phases
of the action
potential.
+30
-, 0
Extracellular- + (Na+)
+
+
+
+
+
fluid
llUl
+
Na+ channel
K+ channel
+
Time
Resting state:
£
At rest, the
membrane is not
-
permeable to Na+ or
K+. Excess positive ions
are on the outside of the
membrane and excess negative
ions are on the inside, creating the
resting membrane potential.
Repolarization
continues:
Na+ and K+ ions are
moved back across the
membrane to help
return the nerve fiber to
the resting potential.
+30 =
0
Time
Q Depolarizing phase:
Following a stimulus,
Na+ channels open and
Na+ enters the nerve
fiber. If the fiber reaches
threshold, a huge influx
of Na+ occurs and the
membrane charges
reverse. This triggers the
action potential.
Repolarizing phase:
The membrane becomes impermeable
to Na+ again and it becomes
permeable to K+.
+30
0
Action Potentials Help Propagate
Nerve Impulses Along the Nerve Fiber
Like muscle cells, neurons are electrically excitable. They
have a voltage difference across their membranes, called
a
resting membrane potential,
which is similar to the
voltage difference across the terminals of a battery. The
resting membrane potential is about -70 mV (significantly
smaller than that of a battery) and is caused by several fac-
tors related to the distribution of electrically charged sodi-
um (Na+) and potassium (K+) ions across the membrane.
The membrane potential of a neuron can change
slightly. When
stimuli
(detectable changes in the internal
or external environment) open or close small ion channels
at dendrites, the membrane potential can become more
positive (depolarization) or more negative (hyperpolariza-
tion). These small changes, called
graded potentials
, can
add together to depolarize the membrane to some critical
level called the
threshold
. Once
a threshold is reached, the mem-
brane potential changes dramati-
cally; various ion channels open
and close in sequence to form an
action potential
(
Figure 7.5
).
The action potential is transmitted rapidly (meters per
second) along the nerve fiber as the inflow of sodium ions
in one portion of the axon stimulates the same sequence of
events in adjacent portions. The action potential is divided
into a depolarizing phase and a repolarizing phase. The re-
polarizing phase of the action potential “resets” the mem-
brane potential so that the neuron can transmit another
action potential. It also limits the transmission of the ac-
tion potential to one direction.
The size of every action potential, called the magni-
tude of depolarization, is always the same. The intensity of
a stimulus is not encoded by the size of the action poten-
tial but rather by the frequency of action potentials. The
greater the stimulus, the more action potentials occur in a
given period of time.
The same sequence of ionic events occurs in unmyelin-
ated and myelinated nerve fibers. However, because action
potentials propagate sequentially down portions of the
membranes of unmyelinated axons and move from node
to node in myelinated axons, myelinated axons transmit
action potentials faster than unmyelinated axons.
Now let's see how an action potential moves from one
cell to another.
action potential
An electrical signal that
propagates along the
membrane of a neuron
or muscle fiber (cell).
Nerve Cells "Talk" to Each Other 197
PROCESS DIAGRAM
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